In the fabrication of articles of manufacture such as film, blow molded articles, and the like from thermoplastic polymers, the fabricator frequently is required to make compromises between optimum physical properties in the ultimate article and the cost and ease of fabrication. This results from the fact that many strength properties such as tensile strength and impact strength increase with the molecular weight of the polymer. The melt viscosity of a polymer also increases with its molecular weight. Thus, if a fabricator prepares the article of manufacture from a higher than normal molecular weight polymer, he must extrude a polymer having higher than normal melt viscosity. This will necessitate either extruding the polymer at a higher than normal temperature, or supplying more than normal power to the extruder, or extruding at a slower than normal rate, or some combination thereof. Any of these adjustments increases the manufacturing cost of the article of manufacture.
The factors discussed in the paragraph above have their principal effect on the cost of manufacturing the desired article of manufacture. In some situations, however, the melt viscosities of certain polymers having molecular weights above specified values are so high as to actually prevent fabrication of articles by melt extrusion processes. This is particularly the case with very high molecular weight linear ethylene polymers having densities greater than about 0.94 gms/ml. The difficulties in extruding such polymers are discussed by L. V. Cancio and R. J. Joyner in their paper GAINS ARE MADE IN EXTRUDING HMW PE POWDERS, Plastics Technology, February 1975, pp 40-44.
Most extrusions of high density linear ethylene polymers are carried out under conditions of smooth laminar flow with the snear stress on the polymer at the die orifice being less than about 3 .times. 10.sup.6 dynes/cm.sup.2*. FNT *The numerical value of shear stress is determined by multiplying the polymer's apparent melt viscosity in poises by the apparent shear rate in reciprocal seconds.
However, when a fabricator attempts to extrude very high molecular weight high density linear ethylene polymers at conventional rates of extrusion, significantly different results are observed. By reason of the polymer's high melt viscosity, the shear stress on the polymer at the die orifice will exceed about 3 .times. 10.sup.6 dynes/cm.sup.2. Under these conditions, the flow of polymer through the die ceases to be smooth and laminar. The extrudate surface becomes rough and nonuniform and the physical properties of the extruded article are reduced significantly. This phenomenon frequently is referred to in the art as melt fracture. The apparent melt viscosities of many very high molecular weight linear ethylene polymers are so high that they simply cannot be extruded at significant rates without encountering melt fracture. For this reason, many very high molecular weight linear ethylene polymers are fabricated by methods other than melt extrusion.